1. Field of the Invention
The present invention relates to a diaphragm control device for a camera, in particular, a diaphragm control device for a camera suited for use in a camera which drives a diaphragm of a camera with a motor.
2. Description of the Related Art
A camera is equipped with a diaphragm mechanism to control light rays from the subject by varying the size of the aperture to record an image of the subject with the proper amount of light.
This diaphragm mechanism functions by connecting with a mirror which moves from the down position to the up position during shooting of a subject.
With the rapid progress of electronic cameras in recent years, cameras which use motors to drive the above-mentioned aperture mechanism and mirror are being proposed.
The diaphragm control mechanism driven by a motor is controlled by the control device to assume a specified diaphragm step number (hereafter AV value).
FIG. 7 is a drawing of a timing chart to control the diaphragm mechanism and the mirror using the above-mentioned control device. Hereinafter control operation of a related art control device will be explained with reference to FIG. 7.
A sequence motor to drive a mirror rotates, by command from the control device, in the normal direction to move the mirror up and in the reverse direction to move the mirror down between first and second positions in and away from an optical axis of said camera.
A sequence switch turns on and off by connection with the motion of the mirror.
A diaphragm pulse is a pulse to transmit the position data of the diaphragm mechanism to the control device.
A diaphragm magnet is a magnetic member acting as a stopping device to stop the motion of the diaphragm mechanism. The magnetic member receives electric current from the control device.
The control device recognizes a delay time from the reception of electric current by the diaphragm magnet to the actual stopping of the diaphragm mechanism as an adjustment value and controls the flow of electric current to the diaphragm magnet as follows.
The control device computes an estimated overrun pulse based on the period of the diaphragm pulse and the above-mentioned adjustment value and controls the flow of electric current to the diaphragm magnet in such a manner that the target diaphragm position is realized ultimately by adding the estimated number of overrun pulses to the diaphragm pulse generated.
However, in the diaphragm control device for a related art camera, the problem occurs that, due to changes in the voltage applied to the motor to drive the diaphragm magnet and the diaphragm mechanism, the time for the diaphragm magnet to stop the diaphragm changes, resulting in a failure to control the diaphragm precisely.
Moreover, even if the voltage applied to the diaphragm magnet is uniform, the acceleration of the diaphragm mechanism during the diaphragm pulses period is not uniform, resulting in a failure to control the diaphragm mechanism precisely.